The present invention generally relates to electrochemical cells. More particularly, the present invention relates to air-assisted alkaline electrochemical cell constructions.
Conventional alkaline electrochemical cells typically include a positive electrode made of manganese dioxide (MnO2), a negative electrode made of zinc (Zn), and an electrolyte made of potassium hydroxide (KOH) in water. The positive and negative electrodes are separated by a separator layer that electrically insulates the two electrodes, while allowing hydroxyl ions to be transferred via the electrolyte between the positive and negative electrodes. During discharge, the MnO2 positive electrode is reduced and the Zn negative electrode is oxidized in what is commonly known as a redox reaction. Thus, during cell discharge, the peroxidation state of the MnO2 positive electrode is lowered as it gives up oxygen. Because conventional primary alkaline electrochemical cells are closed airtight systems, the cell generally becomes expended once the positive electrode has either donated all its available oxygen or the negative electrode has become completely oxidized. Thus, to maximize the service life of the cell, the amounts of active material for the positive and negative electrodes are balanced typically, such that there is a slight excess of positive electrode active material so that gassing is avoided if the cell is forced into deep discharge.
Significant effort has been made to increase the respective amounts of active material that may be contained within an electrochemical cell without expanding the outer dimensions of the cell beyond accepted standard dimensions. Such efforts, however, have not dramatically increased the service life of a primary alkaline cell. Performance improvements in closed-cell batteries are limited by the fact that the cells are closed and that active materials cannot be fed into the cell. To overcome these limitations, air cells were developed that use air as the positive electrode. Such air cells include openings that enable air to pass into and out of the cell, so that oxygen in the air may be used to electrochemically oxidize the negative electrode, which is commonly made of Zn. While such air cells may include a small amount of MnO2 in the region that is exposed to air, the MnO2 is provided primarily to facilitate the electrochemical oxidation of Zn with the air flowing into the cell. The air electrode also contains a conductive material, which provides a path of electrical conduction from the positive contact terminal of the cell. Due to their extended service life, air cells have found use in devices such as hearing aids or the like in which the voltage and current requirements are relatively low. Such air cells typically exhibit open circuit voltages of 1.05 to 1.4 volts. Typical air cells are provided in a coin cell construction. An example of such an air cell is disclosed in commonly assigned U.S. Pat. No. 5,279,905.
Another type of cell is known as the air-assisted cell. Air-assisted cells differ from air cells in that more MnO2 is provided within the cell, such that the MnO2 rather than the air acts as the positive electrode. The MnO2 positive electrode donates hydroxyl ions to the negative electrode in a manner similar to conventional closed-cell alkaline batteries. The air-assisted cells differ, however, from conventional closed-cell alkaline batteries in that air is allowed to pass into the cell and flow over a portion of the positive electrode, so as to recharge the reduced MnO2 electrode by oxidation and hence restore the peroxidation level of the MnO2 in the positive electrode. Because the MnO2 is replenished by the air entering the cell, the Zn negative electrode is the limiting electrode. To maximize the service life of such air-assisted cells, a significant percentage of the internal volume of the cell that was previously dedicated to MnO2 may now be dedicated to Zn while still providing enough MnO2 to produce pulses of the desired current output levels, where air entry is temporarily inadequate. Examples of air-assisted cells are disclosed in commonly assigned U.S. Pat. Nos. 5,270,128, 5,229,223, and 5,079,106.
A significant shortcoming with respect to both air and air-assisted cells results from the fact that they allow ambient air to enter into and exit from the cell, where not only the positive electrode is located, but also the negative electrode and the electrolyte solution. As a result, the Zn in the negative electrode may be wasted by direct oxidation. Further, the air cells and air-assisted cells typically require special consideration when designing the cell construction, specifically the disposition of the negative and positive electrodes relative to one another and relative to any air openings within the cell.
Accordingly, it is an aspect of the present invention to solve the above problems by providing an air-assisted electrochemical cell that minimizes the passage of air to the Zn negative electrode. It is another aspect of the present invention to provide an air-assisted electrochemical cell that enables the freedom to arrange the negative and positive electrodes of the cell so as to maximize performance for any given application. Yet another aspect of the present invention is to provide an air-assisted alkaline electrochemical cell whereby the current density per unit area of the negative/positive electrode interface is significantly decreased, thereby reducing the production of undesirable zinc hydroxide-like reaction products within the cell. It is another aspect of the present invention to provide an alkaline cell in which the maximum energy density and the ampere-hour output is significantly increased.
To achieve these and other aspects and advantages, the electrochemical cell constructed in accordance with the present invention comprises a cell housing having a first end including at least one air opening to allow air to enter into a first portion of the cell housing from the surrounding environment; a first positive electrode provided in the first portion of the cell housing and exposed to the air entering the cell housing through the air opening; a second positive electrode disposed in a second portion of the cell housing; a membrane disposed in the cell housing across the first end between the first and second positive electrodes so as to divide the inside of the cell housing into the first and second portions, the membrane being formed of a material capable of absorbing hydroxyl ions and water while restricting oxygen transmission, so as to allow ion and water transport between the first and second positive electrodes and substantially prevent the air entering the first portion of the cell housing from reaching the second portion of the cell housing; a negative electrode disposed in the second portion of the cell housing; and an electrolyte disposed in the second portion of the cell housing.
Still another aspect of the present invention is to provide a spiral-wound electrode assembly for an electrochemical cell that provides space between the positive and negative electrodes for reaction product to be formed and retained. To achieve this and other aspects and advantages, the electrochemical cell according to a second embodiment of the present invention comprises a cell housing and a spiral-wound electrode assembly disposed in the cell housing, including wound strips of a positive electrode, a negative electrode, and a separator. The spiral-wound electrode assembly further includes a spacer for maintaining a space between the wound strips of negative and positive electrodes for collection and retention of reaction product produced during cell discharge.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.